The aftermath of the tornado that killed Brittany May and several others on Feb. 2, 2007, in Lady Lake, Fla.

She died in her bed.

Brittany May, 17, was asleep at 2 a.m. in Lady Lake, Fla., on Feb. 2, 2007, when a monstrous tornado blew a tree onto her family's mobile home and killed her.

"I've never had to bury a child," her stepmother, Lisa May, said. "I'm going to speak for her mother, and me and her father: This is the hardest thing that any of us will have to do."

Brittany was one of 21 victims of a savage tornado outbreak that slashed across central Florida that month.

Tragically, she was in an unusually vulnerable situation: She lived in a mobile home in the Southeast, during a nighttime tornado outbreak in February. That's the deadliest time and place for twisters, according to Economic and Societal Impacts of Tornadoes, a book just published by the American Meteorological Society.

The USA is heading into the prime months for tornadoes, according to the National Weather Service. The private forecasting firm AccuWeather warns that this spring's tornado season will be more active than normal. AccuWeather meteorologist Paul Pastelok says areas of greatest concern lie from Arkansas and Missouri into Tennessee and Kentucky.

"People are 10 times more likely to die in a mobile home than if the same tornado hit a regular home," says book co-author Kevin Simmons, an economist at Austin College in Sherman, Texas.

Simmons says mobile homes constitute only 7% of the USA's housing stock, but his research found that 43% of all tornado deaths are to people in mobile homes, which can be no match for a tornado's violent winds, clocked as high as 300 mph.

Simmons and co-author Daniel Sutter of the University of Texas-Pan American in Edinburg became interested in tornadoes in 1999, when, living in Oklahoma, they both witnessed one of the most devastating in U.S. history: the May 3 Moore, Okla., twister that killed dozens of people and cost $1.1 billion in damage, according to the Storm Prediction Center.

Such storms are not just about economic damage and resilience by affected communities; they raise a myriad of related issues, such as the effectiveness of Doppler radar, the impact of false alarms and which protective structures are most cost-effective.

The National Weather Service reports that tornadoes kill 56 Americans each year, based on the average from 2000-09, the most recent years for which data are available. This makes tornadoes the fourth-deadliest weather hazard in the USA, after heat, hurricanes and floods.

Roughly 1,200 tornadoes touch down in the USA every year. Other findings from the book:

• The widespread installation of Doppler radar in 1992 "reduced fatalities by almost 40%, and injuries by a similar number," the authors write. "When you consider the value of the avoided fatalities and injuries, the value is over $3 billion. The capital cost to install the radar was less than $2 billion, so this is an investment that was well worth the cost."

• Underground tornado shelters and safe rooms can further reduce casualties beyond that achieved through improved warnings, the authors report. The question of whether people should purchase a shelter is ultimately a personal decision depending on how individuals value safety, they say.

"Our analysis finds that the cost per life that can be expected to be saved with shelters is rather high for permanent homes, even in the most tornado-prone states," Simmons and Sutter write.

"Our analysis does suggest, however, that shelters provide cost-effective protection for mobile homes in those states," the authors write.

• Tornado warnings, issued by the weather service since 1950, are likely saving hundreds of lives in the USA each year. "If there were no tornado warnings, hundreds of Americans likely would die each year in tornadoes, compared to the 50 or so that die now," says Simmons, citing a statistic from tornado expert Harold Brooks of the National Severe Storms Laboratory.

• Increased lead time for tornado warnings has also saved lives. The average lead time before Doppler radar was about five minutes. It is now about 15 minutes.

Nationally, the researchers were surprised that even though three out of every four tornado warnings are "false alarms" — with no tornadoes eventually reported — people still pay attention.

A tornado warning means a tornado has been spotted, or that Doppler radar detects a thunderstorm circulation that can spawn a tornado, the Storm Prediction Center says.

The researchers found that when there has been an unusual number of recent false alarms in a given location, the next outbreak is likely to be more deadly, because tornado warnings do not have the same credibility as when false alarms are rare. And when more people ignore the warning, casualties increase.

• The deadliest states for tornadoes are Kansas, Oklahoma, Arkansas, Mississippi and Alabama. The least deadly? California, Nevada, Oregon, Idaho, Vermont, New Hampshire, and Rhode Island.

• A tornado that occurs overnight is 2½ times more deadly than the same tornado in mid-afternoon.

• Weekend tornadoes are more deadly than a similar storm on a weekday. According to Simmons, it appears that people are safer at work than at home, likely due to the construction practices of commercial buildings as opposed to residential structures. Add to that the fact that many people are involved in activities on the weekend that cause them to be outside and away from shelter.

• The time of the year also matters, as seen in the Florida February 2007 outbreak. So-called "offseason" winter tornadoes like that are more deadly than tornadoes that occur during the traditional "tornado season," which is in April and May.

This is likely due to the fact that people are not as vigilant during the offseason as they are when tornadoes are expected, according to the authors. Therefore, they do not pay as close attention to signals (watching the sky, etc.) as they would during the "tornado season."

Overall, February is the most deadly month.

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